U.S. patent number 10,213,173 [Application Number 14/357,629] was granted by the patent office on 2019-02-26 for whole-body spect system.
This patent grant is currently assigned to KONINKLIJKE PHILIPS N.V.. The grantee listed for this patent is KONINKLIJKE PHILIPS N.V.. Invention is credited to Lingxiong Shao, Herfried Karl Wieczorek, Jinghan Ye.
United States Patent |
10,213,173 |
Wieczorek , et al. |
February 26, 2019 |
Whole-body SPECT system
Abstract
A whole body SPECT system (10) includes a patient support (14)
and a static gantry (12) which includes a plurality of rings
(40a,40b,40c) of radiation detectors (42). The patient support (14)
supports a patient and moves the patient in an axial direction (18)
through the static gantry (12). One or more processors (20,24,32)
connected to the plurality of detectors records strikes of gamma
photons in the radiation detectors (42) and reconstruct the
recorded strikes of the gamma photons into a whole body image.
Inventors: |
Wieczorek; Herfried Karl
(Aachen, DE), Ye; Jinghan (Cupertino, CA), Shao;
Lingxiong (Saratoga, CA) |
Applicant: |
Name |
City |
State |
Country |
Type |
KONINKLIJKE PHILIPS N.V. |
Eindhoven |
N/A |
NL |
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Assignee: |
KONINKLIJKE PHILIPS N.V.
(Eindhoven, NL)
|
Family
ID: |
47561682 |
Appl.
No.: |
14/357,629 |
Filed: |
November 15, 2012 |
PCT
Filed: |
November 15, 2012 |
PCT No.: |
PCT/IB2012/056446 |
371(c)(1),(2),(4) Date: |
May 13, 2014 |
PCT
Pub. No.: |
WO2013/076629 |
PCT
Pub. Date: |
May 30, 2013 |
Prior Publication Data
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Document
Identifier |
Publication Date |
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US 20140343412 A1 |
Nov 20, 2014 |
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Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
Issue Date |
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61562603 |
Nov 22, 2011 |
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Current U.S.
Class: |
1/1 |
Current CPC
Class: |
A61B
6/4258 (20130101); G01T 1/249 (20130101); A61B
6/4266 (20130101); A61B 6/037 (20130101); G01T
1/1644 (20130101); A61B 6/5205 (20130101); A61B
6/4275 (20130101); A61B 6/0487 (20200801); A61B
6/0407 (20130101); A61B 6/4291 (20130101) |
Current International
Class: |
A61B
6/00 (20060101); G01T 1/164 (20060101); A61B
6/03 (20060101); G01T 1/24 (20060101); A61B
6/04 (20060101) |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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58223083 |
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Dec 1983 |
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JP |
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2000206250 |
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Jul 2000 |
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JP |
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2008089396 |
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Apr 2008 |
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JP |
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2004061477 |
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Jul 2004 |
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WO |
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Other References
Beekman, F. J., et al.; Design and simulation of a high-resolution
stationary SPECT system for small animals; 2004; Phys. Med. Biol.;
49:4579-4592. cited by applicant .
Deng, X., et al.; Optimization and Calibration of Slat Position for
a SPECT with Slit-Slat Collimator and Pixelated Detector Crystals;
2011; IEEE Trans. on Nuclear Science; 58(5)2234-2243. cited by
applicant .
Wieczorek, H.; Image quality of FBP and MLEM reconstruction; 2010;
Phys. Med. Biol.; 55:3161-3176. cited by applicant.
|
Primary Examiner: Sunwoo; Nate
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATIONS
This application is a national filing of PCT application Serial No.
PCT/IB2012/056446, filed Nov. 15, 2012, published as WO 2013/076629
A1 on May 30, 2013, which claims the benefit of U.S. provisional
application Ser. No. 61/562,603 filed Nov. 22, 2011, which is
incorporated herein by reference.
Claims
What is claimed is:
1. A nuclear imaging system comprising: a patient support
configured to receive a patient and move the patient in an axial
direction of the patient; a gantry including a plurality of
adjacent stationary circular or elliptical rings of radiation
detectors arranged circumferentially around the patient support in
parallel transaxial planes, each radiation detector on the
plurality of circular or elliptical rings being tilted with respect
to the geometric center of the rings, said tilt being along an axis
that is spaced apart from and parallel to the axial direction of
the patient, differently in the transaxial plane of the circular or
elliptical ring than the radiation detectors at a same radial
position in an adjacent circular or elliptical ring such that
fields of view of adjacent detectors overlap; and wherein the
radiation detectors include slit-slat collimated radiation
detectors, each of the slit-slat collimated radiation detectors
including: at least one slit opening in a sheet of radiation
absorbing material with one side of the sheet surface facing the
patient support and the at least one slit opening extending in the
axial direction, the radiation detectors being circumferentially
offset such that the at least one slit opening of each detector is
circumferentially offset relative to the slit openings of detectors
in the adjacent circular or elliptical ring, a plurality of
parallel slats of radiation absorbing material transaxially
oriented relative to the at least one slit, and an array of
detectors in a plane parallel to the sheet and perpendicular to the
slats; one or more processors connected to the plurality of
detectors and configured to: record strikes of gamma photons in the
radiation detectors, and reconstruct the recorded strikes of the
gamma photons into a single photon emission computed tomography
(SPECT) image.
2. The nuclear imaging system according to claim 1, wherein the
patient support and the gantry undergo continuous movement during
an imaging scan.
3. The nuclear imaging system according to claim 1, wherein the
radiation absorbing material includes at least one of lead and
tungsten.
4. The nuclear imaging system according to claim 1, wherein the
radiation detectors include an array selected from a group
consisting of: (A) at least one scintillator and an array of
silicon photomultipliers; and (B) an array of solid state
detectors.
5. The nuclear imaging system according to claim 1, wherein the
radiation detectors include an array of Cadium Zinc Telluride (CZT)
detectors.
6. The nuclear imaging system according to claim 1, wherein a size
of the gantry is adjustable.
7. The nuclear imaging system according to claim 1, wherein the at
least one of the circular or elliptical rings of radiation
detectors includes an elliptical shape ring.
8. The nuclear imagine system according to claim 1, wherein the
gantry includes at least three circular or elliptical rings of
radiation detectors.
9. The whole body nuclear imaging system according to claim 8,
wherein in each circular or elliptical ring of radiation detectors,
the radiation detectors are circumferentially offset from the
detectors in an adjacent circular or elliptical ring by one third
of a detector.
10. The nuclear imaging system according to claim 1, further
including a display device and wherein the one or more processors
are further configured to control the display device to display the
single photon emission computed tomography image.
11. The nuclear imaging system according to claim 1, further
including: a display device configured to display the single photon
emission computed tomography image.
12. The nuclear imaging system according to claim 1, said tilt
being along the axis of the at least one slit opening of the
radiation detector, said at least one slit opening being spaced
apart from and parallel to the axial direction of the patient.
13. The nuclear imaging system according to claim 1 wherein the
radiation detectors on the plurality of circular or elliptical
rings include some radiation detectors that face the geometric
center of the rings.
14. A nuclear imaging system comprising: a patient support
configured to receive a patient and move the patient in an axial
direction of the patient; a gantry including a plurality of
adjacent stationary circular or elliptical rings of radiation
detectors arranged circumferentially around the patient support in
parallel transaxial planes, each radiation detector on the
plurality of circular or elliptical rings being tilted with respect
to the geometric center of the rings, said tilt being along an axis
that is spaced apart from and parallel to the axial direction of
the patient, differently in the transaxial plane of the circular or
elliptical ring than the radiation detectors at a same radial
position in an adjacent circular or elliptical ring such that
fields of view of adjacent detectors overlap; and wherein the
radiation detectors include slit-slat collimated radiation
detectors, each of the slit-slat collimated radiation detectors
including: at least one slit opening in a sheet of radiation
absorbing material with one side of the sheet surface facing the
patient support and the at least one slit opening extending in the
axial direction, the radiation detectors being circumferentially
offset such that the at least one slit opening of each detector is
circumferentially offset relative to the slit openings of detectors
in the adjacent circular or elliptical ring, a plurality of
parallel slats of radiation absorbing material transaxially
oriented relative to the at least one slit, and an array of
detectors in a plane parallel to the sheet and perpendicular to the
slats.
15. The nuclear imaging system according to claim 14 wherein the at
least one of the circular or elliptical rings of radiation
detectors includes an elliptical shape ring.
16. The nuclear imaging system according to claim 14, said tilt
being along the axis of the at least one slit opening of the
radiation detector, said at least one slit opening being spaced
apart from and parallel to the axial direction of the patient.
17. The nuclear imaging system according to claim 14 wherein the
radiation detectors on the plurality of circular or elliptical
rings include some radiation detectors that face the geometric
center of the rings.
18. A nuclear imaging system comprising: a patient support
configured to receive a patient and move the patient in an axial
direction of the patient; a gantry including a plurality of
adjacent stationary circular or elliptical rings of radiation
detectors arranged circumferentially around the patient support in
parallel transaxial planes, each radiation detector on the
plurality of circular or elliptical rings being tilted with respect
to the geometric center of the rings, said tilt being along an axis
that is spaced apart from and parallel to the axial direction of
the patient, differently in the transaxial plane of the circular or
elliptical ring than the radiation detectors at a same radial
position in an adjacent circular or elliptical ring such that
fields of view of adjacent detectors overlap.
19. The nuclear imaging system according to claim 18 wherein the at
least one of the circular or elliptical rings of radiation
detectors includes an elliptical shape ring.
20. The nuclear imaging system according to claim 18 wherein the
radiation detectors on the plurality of circular or elliptical
rings include some radiation detectors that face the geometric
center of the rings.
Description
The present application relates generally to nuclear medical
imaging. It finds particular application in conjunction with
whole-body single photon emission computed tomography (SPECT), and
will be described with particular reference thereto. However, it
will be understood that it also finds application in other usage
scenarios and is not necessarily limited to the aforementioned
application.
Whole-body SPECT is a nuclear imaging technique in which a
radiotracer is administered to the patient. The radiotracer
typically includes a radioisotope coupled with a biological
substance, such as glucose, with uptake in targeted areas of the
body. The targeted areas include cancer lesions, specific organs,
and the like. Gamma photons are emitted by the radiotracer as the
radiotracer decays, and the emitted gamma photons are recorded by
cameras. The data recorded by the cameras are used to reconstruct
images of the patient showing any lesions in the body.
In order to record the emitted gamma photons from the whole body,
Anger or gamma ray cameras are typically affixed to a rotating
gantry. The bulky and heavy cameras rotate or index about the
patient and record gamma photons. Two cameras, indexed at 32
different angular equi-spaced positions provide good resolution
images. The axial length of the active camera or detector area is
limited. As many as 5 different axial positions are needed to image
the entire body. The rotation to 32 positions for each of 5 axial
positions results in a lot of movement and a long overall time
period to record the entire body. A typical overall time period to
record the entire body is 20-30 minutes or more. A large massive
and expensive gantry is employed to move and hold lead shielded
cameras in precise positions, and permit precise indexing for all
of the positions.
The present application discloses a new and improved whole-body
SPECT system which addresses the above referenced matters, and
others.
In accordance with one aspect, a whole body SPECT system includes a
patient support and a gantry which includes a plurality of
radiation detectors arranged circumferentially around the patient
support. The patient support supports a patient and moves the
patient in an axial direction through the static gantry. One or
more processors connected to the plurality of detectors records
strikes of gamma photons in the radiation detectors and reconstruct
the recorded strikes of the gamma photons into a SPECT image.
In accordance with another aspect, a whole body nuclear imaging
system includes a patient support, a gantry, and one or more
processors. The patient support supports a patient. The gantry
includes a plurality of rings of radiation detectors arranged
circumferentially around the patient support. The patient support
and the gantry are movable relative to each other in an axial
direction. The radiation detectors detect gamma radiation emitted
from the patient supported by the patient support. The one or more
processors are connected to the plurality of radiation detectors
and reconstruct the detected gamma radiation into a whole body
image.
In accordance with another aspect, a method of whole body nuclear
imaging includes, after administering a radiotracer to a patient,
advancing the patient axially through a static gantry which
includes a circumferential arrangement of gamma photon detectors.
Gamma photon strikes are recorded in each detector of the static
gantry. Advancing the patient and recording the gamma photon
strikes are continued until the patient has advanced through the
static gantry. A whole body image is reconstructed from the
recorded gamma photon strikes.
One advantage is the reduced cost.
Another advantage resides in reduced imaging time, by as much as a
factor of 9 to 2-3 minutes for a whole body image.
Another advantage resides in the parallelism offered by a fixed
array of radiation detectors over individual cameras.
Another advantage resides in the elimination of rotational camera
movement.
Another advantage resides in reduced mechanical complexity with the
elimination of a rotating gantry.
Another advantage is the overlap in detectors which increases
efficiency and counteracts patient attenuation.
Still further advantages of the present application will be
appreciated to those of ordinary skill in the art upon reading and
understanding the following detailed description. The invention may
take form in various components and arrangements of components, and
in various steps and arrangement of steps. The drawings are only
for purposes of illustrating the preferred embodiments and are not
to be construed as limiting the invention.
FIG. 1 schematically illustrates an embodiment of a whole-body
SPECT imaging system with a static gantry.
FIG. 2 schematically illustrates one embodiment of the gantry with
a 3 ring configuration of radiation detectors.
FIG. 3 schematically illustrates another embodiment of the gantry
with a 3 ring configuration of radiation detectors.
FIG. 4 illustrates various embodiments of the radiation
detector.
FIG. 5 diagrammatically illustrates an embodiment of one radiation
detector using a slit-slat collimator.
FIG. 6 diagrammatically illustrates an embodiment of a radiation
detector with multiple slits and common slats.
FIG. 7 flowcharts one method of using an embodiment of the whole
body SPECT system.
With reference to FIG. 1, an embodiment of a whole-body SPECT
imaging system 10 with a static gantry 12 is schematically
illustrated. A patient support 14 is mounted on a platform 16 such
that the patient support moves in an axial direction 18 to move the
patient through the static gantry 12. The patient support 14 moves
with a constant axial movement. However, stepping the patient is
also contemplated. The platform 16 can also, optionally, provide
movement up and down to support easier patient loading or access to
the patient support 14. An alternative embodiment holds the patient
support static and moves the gantry axially the length of the
patient.
The static gantry 12 includes one or more rings of radiation
detectors around the patient support 14. The gantry crosses the
patient support under the moving part of the patient support. An
elliptical shape with a wide lateral opening follows the contour of
the body and accommodates heavy patients. In one embodiment, the
ring is 62 cm wide and 46 cm high which includes a 5 cm "keep out"
region or region of space between the patient and the gantry. The
gantry 12 can also have other shapes such as circular. The gantry
can also be adjustable by either adding and removing individual
detectors or by increasing the gantry dimensions and having a
sparse arrangement of detectors.
A data acquisition processor 20 is connected to the radiation
detectors, and receives and records the signals from the plurality
of radiation detectors in a data memory 22. The radiation detectors
record the detector, the locations on the detector, the axial
locations of the patient, and an energy of each gamma photon with
strikes on the detectors as the patient is moved by the patient
support through the eliptical or circular opening in the static
gantry 12. A reconstruction processor 24 reconstructs the data into
a 3D whole body image and stores the image in a memory, such as a
medical records database 26 using a picture and archive
communication system (PACS). In an alternative embodiment, the
radiation detectors communicate wirelessly with the acquisition
processor 20.
An imaging work station 30, which can be local or remote, is in
hardwired or in wireless communication with the medical records
database 26. The imaging work station 30 includes a processor and
memory 32, a display device 34, and at least one input device 36
such as a keyboard, mouse, or microphone. The work station
retrieves the images of the underlying image data and performs
further image enhancements, displays all or a selected portions of
the images, and the like.
FIG. 2 schematically illustrates one embodiment of the gantry 12
with 3 rings 40a, 40b, 40c of radiation detectors 42. Larger and
smaller numbers of rings are also contemplated. The plurality of
radiation detectors 42 can be configured, for example with 52
detectors deposed in each ring. Each detector can be modular such
as a fixed size such as 32.times.32 mm and face the geometric
center of the ring at a pretermined angle, e.g. 90.degree.. The
embodiment includes three rings with the radiation detectors in
each ring differently tilted in a transaxial plane. In one
embodiment, the radiation detectors include slit-slat collimators.
A slit 44 is limited to an opening angle of approximately 70
degrees and extends in the axial direction 18 on each radiation
detector and faces the patient support. Radiation detectors are
located as near as possible to the patient for system efficiency
and spatial resolution leaving a keep out region, e.g. 5 cm between
the patient and the detectors. Larger openings are impractical due
to an increase in penetration through edges and depth-of-interation
effect in flat detectors. Because the slits 44 are located near the
patient, more than one radiation detector 42 angle is used to
provide a total cross section of the patient. To provide multiple
angles, the multiple rings of radiation detectors are used. The
radiation detector angle in each ring is tilted to provide a
complete cross section viewing from within each radial position in
the gantry. The detectors can be circumferentially aligned as shown
in FIG. 2 or can be circumferentially offset, e.g. by one third of
a detector as shown in FIG. 3.
With reference to FIG. 4, various embodiments of the radiation
detector are shown. The radiation detector 42 can be configured
with different types of collimators. One embodiment includes a
slit-slat collimator 46 configuration in which the slit 44 is
paired with parallel slats 48. Alternative embodiments include a
fan slit collimator 50 in which the slit is paired with divergent
slats 48, a single pinhole collimator 54, a multi-pinhole
collimator 56, and the like.
With reference to FIG. 5, one embodiment of one radiation detector
42 uses the slit-slat collimator 46. A sheet 60 of strongly
radiation absorbing metal such as lead or tungsten faces the
patient. In the sheet 60 an opening is formed to define the slit
44. Perpendicular to the facing sheet are parallel slats 48 of
strongly radiation absorbing metal which extend in a transaxial
direction. At the edge of the slats opposite the facing sheet 60
are the gamma photon detectors 70. In one embodiment, the detectors
include one or more scintillation crystals 72 and an array of
photo-sensors 74, such as (analog or digital) silicon
photomultipliers (SiPMs), avalanched photodiodes (APDs),
photodiodes, solid state diodes, or the like. The scintillator 72
can be a single sheet or pixelated. If pixelated, the individual
crystals can be coupled in various configurations with the
photodetectors such as 1:4, 1:1, sparse layouts, offsets and the
like. The crystals produce light when struck by a gamma photon, and
the photo-sensors receive the light scintillations, and generate
electrical signals indicative of the location and energy of each
scintillation. In another embodiment, the scintillator is
eliminated and the photodetectors are replaced with solid state
detectors that convert the received radiation directly to the
signal, such as Cadium Zinc Telluride (CZT) detectors, or the like.
The gamma photon detectors can be arranged in arrays in sizes such
as 1.times.4 mm. The arrays can be grouped into tiles such as a
rectangular arrangement which includes 32 rows of 8 1.times.4 mm
arrays for a total area 32.times.32 mm.
In the illustrated embodiment, the maximum angle of view of the
slit opening 80 is 70 degrees. With the tilt of the radiation
detector along an axis parallel to the axis of the patient in
either direction 82, 84, a complete cross section of the patient is
viewed from each set of radiation detectors. A set of radiation
detectors includes adjacent detectors in adjacent rings which are
tilted differently from the same radial position. This overlap
between radiation detectors leads to a 60% higher efficiency in the
center of the body which counteracts effects of patient
attenuation. The higher efficiency results in better image quality
in the inner part of the patient body. The overlap in range is
optimized so that no region outside the maximum field-of-view for
large patients is seen by the radiation detectors. In an
alternative embodiment, a single ring of detectors can be used
which mechanically rotate back and forth so as to sweep the
field-of-view from each radial position and provide complete cross
section coverage. The closer the detectors are to the surface of
the patient body, the higher the efficiency, and the higher quality
of image resulting. However, space is needed between the radiation
detectors and the patient body as the patient body moves axially
relative to the detectors, and the detectors to view the body
through each slit opening which includes a maximum opening. The
keep out region or space between the patient body and the ring of
detectors provides an optimal balance.
A comparison of features of one embodiment of 3 ring detector
arrangement and one embodiment of a 6 ring detector arrangement is
shown with the low energy high resolution (LEHR) detectors from a
BrightView system in the following table.
TABLE-US-00001 active sensitivity resolution detector area System
(cpm/.mu.Ci) (mm) (cm.sup.2) BV-LEHR x2 390 16 4400 3 rings 306
14.4 1600 6 rings 612 14.4 3200
The sensitivity of the 6 ring system is double the sensitivity of
the 3 ring system at 612 and 306 respectively. The sensitivity of
the 3 ring system is slightly less than the LEHR detectors. The
resolution is constant between the two ring systems and slightly
less than the LEHR detectors. The active detector area for the 6
ring system is double that of the 3 ring system while still less
than the active detector area of the LEHR detectors.
The whole body volume image can be constructed as a stack of planar
images. A planar image can be take from arbitrary angles and
arbitrary selected parts of a volume data. Detectability is
improved over traditional planar imaging through the ability to
move through slices of the volume in viewing locations of
lesions.
Detectability is determined by the contrast to noise ratio (CNR),
according to the Rose criterion. CNR is defined as the difference
between lesion and background signal integrated over the lesion
area and divided by the background noise integrated over an
equivalent area.
An example uses a cylindrical body of 400 mm diameter, a lesion of
16 mm diameter, and a contrast C.sub.0 in the center. The radii,
given in units of voxels with 4 mm size, are r.sub.b=50 for the
background radius and r.sub.1=2 for the lesion radius. A CNR of a
theoretical cut-out transaxial slice of the body is
CNR.sub.0=C.sub.0A.sub.bTEr.sub.i.sup.2.pi./ {square root over
(A.sub.bTEr.sub.1.sup.2.pi.)}=C.sub.0 {square root over
(A.sub.bTEr.sub.1.sup.2.pi.)}, where A.sub.b represents background
activity, T is the total imaging time, and E is the system
efficiency. Using an analytical model derived for filtered
back-projection and Q=0.056 for the Hann filter with linear
interpolation, the CNR is calculated for planar imaging and
reconstructed slices. To adjust for the lower noise of statistical
reconstruction, a factor of 2 is included which replaces 1/ Q by a
factor of 6. The CNR is calculated and show relative to each other
using a planar image (CNR.sub.p), a reconstructed central slice
(CNR.sub.r), all slices summed in a volume (CNR.sub.v), and for a
lesion volume (CNR.sub.1). CNR.sub.p=CNR.sub.04r.sub.1/(3 {square
root over (2r.sub.b)})=CNR.sub.00.27 CNR.sub.R=CNR.sub.01/ {square
root over (2r.sub.bQ)}=CNR.sub.00.6
CNR.sub.V=CNR.sub.02r.sub.1/(3r.sub.b {square root over
(Q)})=CNR.sub.00.16 CNR.sub.L=CNR.sub.02/3 {square root over
(r.sub.1/(r.sub.bQ))}=CNR.sub.00.8 The example shows that the CNR
for a single slice (CNR.sub.r) or multiple slices in the interval
of (0.27, 0.6] is better than planar imaging (CNR.sub.p). Comparing
the example of the lesion volume to the planar image gives a
0.8/0.27 or approximately 3 times the CNR which can be used to
reduce imaging time by a factor of 9. Reducing the imaging time by
a factor of 9 can reduced the overall image time from more than 20
minutes to approximately 2-3 minutes.
FIG. 6 diagrammatically illustrates an embodiment of a radiation
detector with multiple slits 44 with common slats 48. Multiple
rectangular gamma photon detectors 74 such as SiPM tiles are
located proximate to each slit.
With reference to FIG. 7, one method of using an embodiment of the
whole body SPECT system is flowcharted. A patient is loaded on the
patient support 14 and administered a radiotracer according to the
radiotracer protocol. The patient is advanced axially 18 through
the opening in the static gantry 12 in a step 90. In step 92, the
detector, the location on the detector, the patient location, and
the energy level for each gamma photon strike is recorded as the
patient passes through the field of view. Using all the radiation
detectors in the static gantry, the entire cross section of the
patient is recorded without movement of the detectors in the
gantry. The process continues in step 94 until the patient has
advanced completely through the static gantry. In step 96, a whole
body image is reconstructed using the recorded gamma ray strike
data. The volume image is stored in the medical records database
26. The work station 30 is used to display the whole-body image or
a portion of an image such as a planar image, a surface rendering
or the like. Oblique slices can be presented from any of multiple
angles.
The method described can be implemented using one or more
processors executing one or more computer readable instructions
encoded on a computer readable storage medium such as physical
memory which causes the one or more processors to carry out the
instructions. Additionally or alternatively, the one or more
processors can execute instructions carried by transitory mediums
such as a signal or carrier wave.
The invention has been described with reference to the preferred
embodiments. Modifications and alternations can occur to others
upon reading and understanding the preceding detailed description.
It is intended that the invention be constructed as including all
such modifications and alterations insofar as they come with the
scope of the appended claims or the equivalents thereof.
* * * * *